The invention relates to a method, implant, and apparatus for altering the curvature of the cornea of the eye for the correction of refractive errors.
It is well known that refractive errors can be corrected by reshaping the cornea of the eye to obviate the need for glasses or contact lenses. Reshaping of the cornea may be accomplished in several ways. Prior art techniques of refractive surgery are discussed in detail in the text entitled "Refractive Keratotomy for Myopia and Astigmatism" by George O. Waring III published by Mosby Year Book of St. Louis, Mo., 1992. For example, the well known radial keratotomy method for correction of myopia involves the creation of multiple radial incisions through epithelium, Bowman's membrane, and at least 90% of the stromal thickness. This results in marked weakening of the structural integrity of the midperipheral cornea, causing relative steepening in this region and relative flattening of the central corneal curvature. This method has been associated with a number of undesirable factors including undercorrection or overcorrection of refractive error, fluctuating vision, corneal rupture from trauma, regression or progression of refractive effect, inability to correct high myopia, and infection of corneal incisions.
Another method of reshaping the cornea involves the use of lasers to ablate portions of the anterior surface to precise depths, as disclosed in U.S. Pat. No. 4,732,148. This method has also been associated with undercorrection or overcorrection of refractive errors, creation of haze or scarring within the visual axis, regression of refractive effect, a prolonged healing period, and requires use of expensive equipment. U.S. Pat. No. 4,461,294 describes the use of laser energy to vaporize light-absorbing pigments or dyes injected into the corneal stroma to produce multiple radial corneal scars. The corneal scars contract along their length resulting in flattening of central corneal curvature for the correction of myopia. However, variable wound healing following this procedure might result in unpredictable refractive effect.
Intracorneal synthetic or biological implants have been described in the prior art for alteration of corneal curvature. For example, U.S. Pat. No. 4,607,617 describes the implantation of synthetic lenses within the central corneal stroma for correction of refractive errors. These implants and methods have been associated with undesirable side effects including necrosis of tissue anterior to the implant due to interference with nutrient transfer, irreversible scarring and vascularization within the visual axis, undercorrection and overcorrection of refractive error, and the surgery can be technically difficult to perform.
U.S. Pat. No. 5,300,118 and related patents describe the implantation of a single intrastromal, circumferential, biocompatible ring of varied thickness for the correction of refractive errors. This implant exerts a direct conformational change upon the cornea resulting in flattening of the central cornea in proportion to the thickness of the ring. However, this prior art differs from the present invention in several ways, including 1) the present invention utilizes an implant of much smaller dimensions and of a completely different shape, 2) the implants of the present invention are placed radially within the cornea instead of circumferentially, 3) only the present invention allows adjustment of refractive effect by application of heat to the implants, 4) the radial placement of the implants of the present invention is technically easier to perform and is less error-prone than the circumferential placement of the prior art method, 5) the radial placement of the implants of the present invention exert different stress vectors on the cornea than the prior art, 6) the prior art uses one or two ring-shaped implants always placed in the same corneal location whereas the present invention uses 1 to 30 implants placed in a variety of locations, corneal depths, and distances from the central cornea depending on preexisting refractive error, 7) the prior art uses implants all of the same initial curvature whereas the present invention uses implants of various preselected curvatures, 8) the present invention allows more selective alteration of refractive effect by removing and replacing individual small implants which only affects the corneal curvature immediately surrounding the implant instead of large portions of the cornea with the prior art, and 9) the prior art method requires a very different and more complex apparatus to create the corneal incisions for the implant.
U.S. Pat. No. 4,781,187, a modification of the radial keratotomy procedure, describes the suturing of donor corneal tissue implants of rectangular or triangular geometry within conventional radial keratotomy incisions. This method provides variable change in corneal curvature by controlling the amount of gape within radial keratotomy incisions. The implants described in this patent are relatively nonrigid and do not directly exert conformational changes upon the surrounding cornea. These implants maintain the gaping of radial incisions at the corneal surface, thereby increasing the circumference of the mid-peripheral cornea. This prior art differs from the present invention in several ways including the requirement of deep corneal incisions extending to the outer surface of the cornea and is associated with undesirable factors such as difficulty of producing and handling very thin donor corneal implants of precise thickness, and overcorrections or undercorrections due to variable wound healing and variability of suture tightness. The wound healing variability associated with conventional radial keratotomy would also apply to this prior art procedure.